I. Introduction

This report explores several characteristics of juvenile snowshoe hares (Lepus americanus) live-trapped as part of a long term ecological research study at the Bonanza Creek Experimental Forest. Hares were trapped from 1999-2012 at 3 different sampling sites, or “grids”. The sites included a flood zone along the Tanana River (Bonanza Riparian), mature stands of white spruce, paper birch, alder, black spruce, Labrador tea, willows, and prickly rose (Bonanza Mature), and a mature lowland black spruce stand (Black Spruce). Researchers recorded the date each hare was trapped as well as physical measurements for each (Kielland et al., 2017). The following sections include visualizations and analyses on juvenile hares only, comparing trap numbers per year , their weight by sex and site, and displaying the relationship between hind foot length and individual weight.

II. Data and Analysis

Dr. Knut Kielland and colleagues collected these data to explore population densities in boreal regions of northern Alaska with the Bonanza Creek LTER (long term ecological research)(Kielland et al., 2017). The data contains physical measurements for hare ear size, weight, sex, age, and hind foot length. First, the report visualizes hares trapped per year and the weight of male hares versus female hares at each site. It goes on to compare male and female hare weight using an unpaired t-test (significance level of 0.05) and Cohen’s d effect size. The relationship of hind foot length and weight is explained through a simple linear regression and Pearson’s r correlation. All analyses are in R version 4.1.1 using RStudio.

III. Juvenile Hare Initial Investigation

A. Annual juvenile hare trap counts

Figure 1: Annual juvenile hare trap counts from 1999-2012. Height of the bar indicates the amount of hares trapped that year at all 3 sites combined.

The data visualization clearly shows that there is a large variation between years in the number of juvenile hares that were collected. The max was 126 while the minimum was only 2, excluding the years there were no data. In general, there is a larger number of hares collected in the first few years of the study than in the last. In fact, the first year of the study had the highest amount of hares trapped. The mean number of juvenile hares trapped over the 13 year period is 31.5 while the median is only 18.5. In moving forward, I would make sure that the people collecting data know that it is crucial to have uniform sampling effort from year to year. This means sampling at similar times each month with the same amount of traps set and the same amount of days. If the effort varies from year to year, the data will no longer reflect actual sample size.

B. Visualize juvenile hare weights

Figure 2: Weight observations (in grams) for juvenile hares taken at 3 different sites. The sites include a mature lowland black spruce stand (Black Spruce), stands of mature white spruce, paper birch, and alder, as well as black spruce (Mature), and flood zone along the Tanana River (Riparian). Females are represented by the yellow points and male represented by brown. Hares trapped, but with no recorded sex, are represented as gray points. The black points represent the mean number of hares for each sex.

This graph displays the distribution of hare weight according to sex and site. It can be observed that there were a lot more hares trapped in the riparian site than the other two sites. In addition, one can see that at every site, the male mean is greater than the female mean, leading us to the conclusion that male hares, in accordance with normal sexual dimorphism patterns, are usually larger. It may also be important to observe that the means for all 3 sites fall within a close range of 800-1100 grams.

C. Juvenile weight comparison (male and female snowshoe hare)

Table 1: Summary statistics for male and female hares captured from 1999-2012
Sex Mean weight (g) Standard deviation (g) Sample size
female 855.39 292.25 200
male 945.86 333.22 163

The means of males versus females were compared using a few different statistical tests. Initially, the actual difference in mean was calculated to be 90.4680328 (grams). This number alone may not be enough to really tell if the means are statistically different. Upon running a t-test, the p-value returned as 0.0070933. This means that there is only a 0.7% chance that if you take 2 samples from populations with the same means the difference would be at least this different. This would normally lead one to believe that the male and female weights are sigificantly different, however the effect size was also calculated using the Cohen’s d test. This rendered an effect size of 0.2904674 which is interpreted as small. While the statistical t-test renders the means different, the effect size is small meaning the difference is negligible.

D. Relationship between juvenile weight and hind foot length

Figure 3: Hindfoot length (mm) versus weight (g) in juvenile snowshoe hares.

Summary

IV. Citation

Data citation: Kielland, K., F.S. Chapin, R.W. Ruess, and Bonanza Creek LTER. 2017. Snowshoe hare physical data in Bonanza Creek Experimental Forest: 1999-Present ver 22. Environmental Data Initiative. https://doi.org/10.6073/pasta/03dce4856d79b91557d8e6ce2cbcdc14